Circuits which drive relays are generally circuits which maintain a contact by applying a constant voltage to a relay coil by means of a constant-voltage power supply.
However, when a relay is driven under constant voltage, the following problem is known to arise. Normally a relay contact is held closed by the magnetomotive force arising in the relay coil within the relay due to current flowing in the relay coil. This magnetomotive force is determined by the current flowing in the relay coil and by the number of coil turns. On the other hand, the relay coil generates heat due to losses which are the product of the resistance and the flowing current. Due to this heat generation the coil resistance increases, the current flowing in the coil decreases, and the magnetomotive force declines; and so a constant voltage sufficient to obtain the magnetomotive force necessary to hold the contact closed even after the increase in coil resistance is applied to the relay coil as the driving voltage.
Moreover, heat generation by the relay coil has a large effect not only on the relay lifetime, but also on peripheral equipment, and suppression of heat generation is an important factor of relay control. However, due to the counter-electromotive force in the relay coil occurring due to the sharp increase in coil current immediately after the driving voltage is applied to operate the relay, the magnetomotive force is reduced, and so the driving voltage must be increased in order to compensate for the coil counter-electromotive force. Hence it is desirable that during relay operation, the relay driving voltage be increased to cause the relay to operate reliably, and that after the relay is in the operating state and has stabilized, that the minimum driving voltage necessary to hold the contact closed by applied, in order to suppress heat generation by the relay.
As technology to reduce the driving current and suppress heat generation while causing a relay to operate reliably, technology to provide one constant-voltage power supply and to limit the current flowing in the coil by means of a series resistor after applying the output voltage of the constant-voltage power supply to the coil (see for example Patent Documents 1 and 2), and technology to lower the driving voltage by dividing the voltage applied to the coil using a Zener diode connected in series with the coil (see for example Patent Document 3), are known. And, technology to comprise a constant-voltage power supply which outputs a high voltage for initiation of relay operation and a constant-voltage power supply which outputs a low voltage to maintain the relay contact in the operating state, and to selectively switch the constant-voltage power supplies at the time of initiation of relay operation and after initiation of operation (see for example Patent Document 4), as well as technology to reduce coil losses by executing on/off control of the driving voltage in pulse form (see for example Patent Document 5), and similar are known.
In this Specification, in conformance with the definition of terms in JIS (Japan Industrial Standards) C4530-1996, relay “operation” indicates a transition of the relay from the reset state to the set state, “reset” indicates a transition of the relay from the set state to the reset state, “set state” indicates a state in which all form b contacts are open, all form a contacts are closed, and the relay is mechanically stable, and “reset state” indicates a state in which all form a contacts are open, all form b contacts are closed, and the relay is mechanically stable.
However, in the case of the technology of Patent Documents 1 and 2, because of the resistor connected in series with the relay coil, a loss occurs due to the product of the coil driving current and the resistance of the resistor, and in the case of the technology of Patent Document 3, because of the Zener diode connected in series with the relay coil, a loss occurs due to the product of the coil driving current and the Zener voltage of the Zener diode, so that there are the problems of increases in energy losses and heat generation. And in the case of the technology of Patent Document 4, because two constant-voltage power supplies are necessary, there is the problem that the circuit scale is increased. And in the case of the technology of Patent Document 5, there is the problem of the occurrence of radiation noise upon on/off control of the relay driving voltage.    Patent Document 1: Japanese Patent Application Laid-open No. H10-255627    Patent Document 2: Japanese Patent Application Laid-open No. 2005-268134    Patent Document 3: Japanese Patent Application Laid-open No. 2000-113787    Patent Document 4: Japanese Patent Application Laid-open No. 2005-38656    Patent Document 5: Japanese Patent Application Laid-open No. 2006-114446